Current Ratio Transformer

Current Ratio Transformer
[instructions for use]
Avo Ltd, Dover, England
a member of the Thorn Group
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[Rochester's labels]
Cal Period 12 months
Calibrated 14-1-08
Instrument number ETL 977
Certificate number 080058
Calibration due 14-1-09
BAE Systems
AS E&ETC
AS24 Insp.

This is a high power current transformer used for current measurements and injection tests as part of EMC conducted emissions and susceptibility testing. The object is marked ‘Instrument No. ETL 977’, ‘Mark V’. It has selectable Ratios from 25:1 (30A max) to 400:1 (600A max).

A Current Transformer is a type of “instrument transformer” that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line using a standard ammeter.

The principle of operation of a basic current transformer is slightly different from that of an ordinary voltage transformer. Unlike the voltage or power transformer the current transformer consists of only one or very few turns as its primary winding. This primary winding can be of either a single flat turn, a coil of heavy-duty wire wrapped around the core or just a conductor or bus bar placed through a central hole as shown.
See: Electronics Tutorials - The Current Transformer.

The date of this transformer will be prior to 1987 when AVO became Megger, but the exact date is unknown.